1. Field of the Invention
The present invention relates to a broadband wireless access (BWA) communication system. More particularly, the present invention relates to an apparatus and method for reducing the required amount of memory by using a memory efficiently when using a hybrid ARQ (HARQ) scheme for increasing the reliability of data transmission.
2. Description of the Related Art
Extensive research is being conducted to provide various Quality of Service (QoS) features with a data rate of about 100 Mbps in the advanced fourth-generation (4G) communication system. The 4G communication system is evolving to provide mobility, high data rate transmission, and high QoS in a BWA communication system such as a wireless local area network (WLAN) system and a wireless metropolitan area network (WMAN) system. Typical examples of the above system are identified in the Institute of Electrical and Electronics Engineers (IEEE) 802.16d system and the IEEE 802.16e system standards. WiMAX or WiBro uses the communication technologies of IEEE 802.16d and IEEE 802.16e.
The IEEE 802.16d system and the IEEE 802.16e system use an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme to provide a broadband transmission network for a physical channel of the WMAN system. The IEEE 802.16d system considers only a fixed subscriber station (SS) and a single cell structure (for example, the mobility of an SS is not considered). The IEEE 802.16e system considers the mobility of an SS. When the mobility of an SS is considered, the SS will be referred to as a mobile station (MS). Hereinafter, an SS and an MS will be collectively referred to as ‘user equipment (UE)’.
Because a signaling communication between a base station (BS) and a UE is performed through a direct link, the general IEEE 802.16e system can easily provide a highly reliable wireless link between the BS and the UE. However, because the BS is stationary, the IEEE 802.16e system has a low flexibility in constructing a wireless network. Accordingly, the IEEE 802.16e system makes it difficult to provide an efficient communication service in a radio environment where traffic distribution or call requirements change frequently. In order to overcome this problem, a stationary relay station (RS), a mobile RS or general UEs can be used to apply a multi-hop relay data transmission scheme to a general cellular communication system such as the IEEE 802.16e system. The use of the multi-hop relay scheme can expand a coverage area of the BS.
FIG. 1 is a schematic block diagram of the IEEE 802.16e system that is a general BWA communication system.
Referring to FIG. 1, the IEEE 802.16e system has a multi-cell structure. The IEEE 802.16e system includes a cell 100, a cell 150, a BS 110 managing the cell 100, a BS 140 managing the cell 150, and a plurality of UEs 111, 113, 130, 151 and 153. The signal exchange between the BSs 110 and 140 and the UEs 111, 113, 130, 151 and 153 is performed using the OFDM/OFDMA scheme. When the UE 130 is located in a boundary region (for example, a handover region) between the cells 100 and 150, it may perform a handover due to its moving direction or other reasons. That is, when the UE 130 moves from the cell 100 of the BS 110 into the cell 150 of the BS 140 while communicating with the BS 110, the serving BS of the UE 130 is changed from the BS 110 to the BS 140.
In order to increase the reliability of data transmission, an HARQ scheme is used to perform data transmission between the BS and the LTE in the BWA communication system.
The HARQ scheme performs error detection on data received by a receiver, and transmits an ACKnowledgement (ACK) signal or a Negative ACKnowledgement (NACK) signal to a transmitter according to the error detection results. For example, if there is an error in the data, the transmitter retransmits the data to the receiver. Upon receipt of the retransmitted data from the transmitter, the receiver combines the retransmitted data and the previously-received data, thereby increasing the reliability of data transmission.
In order to combine the retransmitted data and the previously-received data, the receiver needs a memory for storing the previously-received data.
An HARQ scheme in a WiMAX profile defines four-level parameters depending on the buffer capacity. Table 1 shows the memory sizes required at the respective levels.
TABLE 1DownlinkLevelAggregationMemory Size1OFF16,384 bits × 4 (LLR bits) × 4channels = 262,144 bits2ON8,192 bits × 16 channels × 4 (LLR bits) × 16channels = 8,388,608 bits3ON16,384 bits × 16 channels × 4 (LLR bits) × 16channels = 167,77216 bits4ON23,170 bits × 16 channels × 4 (LLR bits) × 16channels = 23,726,080 bits
Referring to Table 1, an aggregation at a Level 1 is basically off, while aggregations at Levels 2, 3 and 4 are basically on.
In Table 1, the log likelihood ratio (LLR) bit denotes the resolution of bits indicating a data symbol in a receiver, which is an implementation-dependent value. In general, the LLR bit has a bit resolution of about 4˜10 bits.
In Table 1, the bit resolution is assumed to be 4 bits, in order to illustrate a relative value at each level and a buffer size at each level in the HARQ scheme. A memory of the total memory size is required for each ARQ channel ID (ACID) in order to support the aggregation. The ACID indicates an input data channel.
If the aggregation is on, a downlink (DL) burst with a size corresponding to the total channel capacity (for example, 16 channels for Level 3) can be transmitted to one ACID. That is, if the aggregation in Level 3 is on, a memory with a size of 16 bits is required.
Thus, if the aggregation is on, a buffer with a memory size defined in Table 1 is required for each level. What is therefore required is an apparatus and method for managing the buffer efficiently.